Sect. 64 describes a mixed number as being composed of all numerals. It says no letter may be used. OK. But it doesn't mention substituting an omission for one or more of the numerals.
The Intro to Brl Math, in sect 74, specifically says the omissions can be used. Is this authoratative? This is one of those areas where you can go along happily for years and then suddenly question it. I always go with a true mixed number in such a case.